1. Field of the Invention
The present invention relates to a wireless base station, a wireless communication terminal, and a wireless communication system, and more particularly to a wireless base station, a wireless communication terminal that establishes connection with the wireless base station, and a wireless communication system which perform communication in a spot communication area.
2. Description of the Related Art
In recent years, portable equipment equipped with a communication function that meets the IEEE802.11 standard (hereinafter referred to as a wireless LAN communication function) is increasingly becoming widespread. In addition, an area of use is not limited to an office or a house, but areas usable as a so-called hotspot are rapidly increasing outdoors, although the area is an isolated spot communication area. In response to this, wireless LAN is not only limitedly mounted on such apparatuses as a personal computer or office equipment, but is also mounted to various apparatuses, including household electrical appliances. Particularly, the wireless LAN is increasingly used in various portable-type or in-vehicle type apparatuses such as a mobile phone.
In the IEEE802.11 standard, in order to establish connection between a communication terminal and a wireless LAN base station, the following basic process is required. First, the communication terminal scans all communication channels so as to receive beacons periodically sent by wireless LAN base stations. Next, the communication terminal specifies communication channels of the wireless LAN base stations, determines a wireless LAN base station desired to be connected, and decides whether or not connection with the wireless LAN base station is permissible by using an identifier, called an SSID (Service Set Identifier). When connection is permissible, a communication path between the wireless LAN base station and the communication terminal is encrypted by using an encryption format typified by a WEP (Wired Equivalent Privacy), and a WPA (Wi-Fi Protected Access). Upon completion of recognition and encryption, the communication terminal finally becomes capable of starting communication with the wireless LAN base station.
In this manner, the IEEE802.11 standard has a feature that communication is started upon completion of a series of connection processes including communication channel scanning, detection of a wireless LAN base station, authentication, and encryption. Two modes are defined as communication methods, that is, a unicast mode in which information is transmitted to specific communication terminals only by registering addresses of the communication terminals as destinations of communication packets; and a broadcast mode in which information is transmitted to an indefinite number of communication terminals by registering a broadcast address as the destination.
In the future, as a service using a wireless LAN-mounted apparatus, it is expected that a service having the following system will be increasingly widely used. That is, wireless LAN base stations are located outdoors, and when a communication terminal passes through a spot communication area of one of the wireless LAN base stations, information is transmitted/received between the communication terminal and the wireless LAN base station. In the future, the communication terminal will not be merely used in a semi-fixed state, but will be used during walking. In addition, it is expected that the communication terminal will be increasingly used for high-speed movement such as movement by bus or by train, or used for a car navigation system equipped with a wireless LAN communication function. As the movement speed of the communication terminal is increased, there is need for reduction in time elapsing from start of communication between the communication terminal and the wireless LAN base station to reception of contents from a server by the communication terminal, in addition to reduction in time necessary for a user operation. For example, many communication terminals are connected to a wireless LAN wireless base station, a limited communication band is shared by a plurality of communication terminals, and consequently a communication rate for each communication terminal is lowered. As a result there may be a case where a communication terminal hardly transmits/receive information during passing through a spot communication area.
It is expected that the number of communication terminal users will be increased. Moreover, it is expected that data transmitted between an individual communication terminal and a wireless LAN base station will be dominated by such data having large capacity as audio data and visual data rather than text only data. In order for a plurality of communication terminals to receive such information during passing through the spot communication area, it has been needed a communication method that exerts superior transmission efficiency.
Various countermeasures have been considered against the above-described problem. For example, disclosed in Japanese Laid-Open Patent Publication No. 2004-048395 (hereinafter, referred to as Patent Document 1) is a portable information terminal wireless LAN service so as to provide a system that distributes information to a portable information terminal passing through a communication area of a wireless LAN base station. FIG. 40 is a block diagram illustrating a configuration of a conventional portable information terminal wireless LAN service system disclosed in patent document 1. In FIG. 40, suppose that a portable information terminal (PDA) 2001 is owned by a user who, in advance, subscribes to an information service through a mobile phone, a PHS, or the like.
The PDA 2001 has a WOR (Wake-on Ring: automatic activation) function, and in addition, has a wireless LAN client software installed thereon. An information provider 2004 is connected to a wake-on server 2005 and a gate server 2002 which are controlled by an agent server 2003. In a spot communication area where information is provided, a wireless LAN base station (not shown) is located so as to perform data transmission/reception with a PDA 2001 owned by a user. When a user owning the PDA 2001 passes through the spot communication area, the wake-on server 2005 causes client software of the PDA 2001 to start. The client software of the PDA 2001 is connected to the gate server 2002 via the wireless LAN base station, and information distributed from the information provider 2004 is stored in a memory of the PDA 2001.
However, even if the technology disclosed in Patent Document 1 is used, when it is assumed that a communication terminal moves in a spot communication area or the like at a high speed, time taken by the communication terminal to receive contents has not been sufficiently reduced. In Patent Document 1, by having the client software started by the wake-on server 2005, the user operation necessary for connection to the wireless LAN base station has been improved. However, whether or not the communication terminal has improved communication efficiency and whether or not time elapsed until the contents are received are not described therein.
As to the improvement in the communication efficiency, as described above, the broadcast mode is defined under the IEEE802.11 standard, and it is possible to transmit information simultaneously to a plurality of communication terminals. When the broadcast mode of the wireless LAN is used, the information transmission efficiency is improved, however, the base station is to transmit the same information to the plurality of communication terminals regardless of whether or not the communication terminals have requested for such transmission. Since memory capacity of the base station is limited, there is a possibility that contents desired by a communication terminal will not be transmitted thereto. On the other hand, since information requested by an indefinite number of communication terminals is transmitted, unnecessary pieces of information are also broadcasted. As a result, efficiency for transmitting a piece of information requested by a communication terminal is lowered.